Accelerometers are well known in the art. An accelerometer is a device which measures acceleration, or more accurately measures force exerted by a body as a result of a change in the velocity of the body. A moving body possesses an inertia which tends to resist change in velocity. It is this resistance to change in velocity that is the source of the force exerted by the moving body. This force is directly proportional to the acceleration component in the direction of movement when the moving body is accelerated.
Various types of accelerometers are available. Generally, in a micromachined accelerometer formed using silicon, a central (e.g., typically spherical or rectangular shaped) mass is suspended by one or more microbridges. The microbridges are attached to a supporting substrate which surrounds the mass with a gap provided therebetween. The mass is supported within and has free movement relative to the supporting structure.
The movement of the mass is measured in various manners. For example, the movement of the mass may be measured by measuring a corresponding change in the output of a wheatstone bridge incorporating beam piezo resistors formed in the microbridges.
Further, for example, in silicon capacitive accelerometers, such as those available from VTI Hamlin (Finland), the sensing element of the accelerometer consists of three layers of silicon isolated from each by thin glass layers. The middle silicon layer incorporates a cantilevered mass beam structure. The force of gravity or acceleration acting on the silicon mass causes the beam structure to bend. This deflection is detected as a change in the distance between electrodes in capacitors formed on both sides of the mass using metal electrodes.
Generally, such micromachined accelerometers require external circuitry to process the signal output by the accelerometer. For example, such a signal output may be used for triggering an automobile airbag of an airbag deployment system, may be used for triggering medical treatment in an implantable medical device, or may be used as an input for any other application where acceleration is to be detected.
Accelerometers are generally constrained in that typically a micromachined accelerometer as described above has a single axis sensitive to acceleration. That is, the sensing element of an accelerometer can only measure acceleration along a line perpendicular to a particular plane thereof. For example, the plane may be defined by a principle surface of the sensing element from which side during fabrication various fabrication steps are performed, e.g., masking, etching, etc. For example, in an automobile airbag system, the direction of acceleration which must be sensed in the event of a collision is typically along a line lying in a horizontal plane, i.e., parallel to the ground. Further, for example, the direction of acceleration which is to be sensed of a person implanted with an implantable medical device, may be the direction of acceleration along a line lying in a horizontal plane, i.e., a line orthogonal to a plane defined by the patient's chest.
Various accelerometer structures are available such that the accelerometer can be surface mounted on a substrate in one or more orientations. For example, an accelerometer available from EG&G Inc., (Wellesley, Mass.), available under the Model No. 3255 includes an accelerometer wherein the sensitive axis is perpendicular to the bottom plane of the package. The package can be mounted in two orientations, allowing the sensitive axis to be either perpendicular or parallel to the mounting plane defined by the substrate upon which it is mounted. This accelerometer measures acceleration using a wheatstone bridge technique.
Table 1 below lists U.S. Patents showing other transducer, e.g., accelerometer, configurations.
TABLE 1 U.S. Pat. No. Inventor(s) Issue Date 4,896,068 Nilsson 23 January 1990 5,044,366 Alt 3 September 1991 5,215,084 Schaldach 1 June 1993 5,425,750 Moberg 20 June 1995 5,674,258 Henschel et al. 7 October 1997 5,373,267 Kaida et al. 13 December 1994 5,318,596 Barreras et al. 7 June 1994 5,031,615 Alt 16 July 1991 4,653,326 Danel et al. 31 March 1987 5,594,172 Shinohara 14 January 1997 4,140,132 Dahl 20 February 1997 4,679,434 Stewart 14 July 1987 4,742,182 Fuchs 3 May 1988 4,891,985 Glenn 9 January 1990 4,987,7810 Reimann 29 January 1991 5,014,702 Alt 14 May 1991 5,031,615 Alt 16 July 1991 5,235,237 Leonhardt 10 August 1993 5,745,347 Miller et al. 28 April 1998 5,616,863 Koen 1 April 1997 5,503,016 Koen 2 April 1996
All references listed in Table 1, and references listed elsewhere herein, are incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, at least some of the devices and methods disclosed in the references of Table 1, and elsewhere herein, may be modified advantageously by using the teachings of the present invention. However, the listing of any such references in Table 1, or elsewhere herein, is by no means an indication that such references are prior art to the present invention.